Spectral Shaping of Pseudorandom Binary Sequence

ABSTRACT

A pseudorandom binary sequence having a first sampling rate is generated. Based on the pseudorandom binary sequence, a digital filter generates a spectrally shaped pseudorandom binary sequence having a second sampling rate. The second sampling rate is equal to the first sampling rate multiplied by an integer upsampling factor of L&gt;1. The digital filter includes L filter branches consisting of a first subset of one or more filter branches and a second subset of one or more filter branches. Each filter branch of the first subset generates a binary output which is equal to an input of the filter branch. Each filter branch of the second subset generates a binary output which is inverted with respect to an input of the filter branch.

TECHNICAL FIELD

The present application relates to generation of a spectrally shapedpseudorandom binary sequence and to devices in which a spectrally shapedpseudorandom binary sequence is generated.

BACKGROUND

For various kinds of applications, it is known to utilize a pseudorandombinary sequence. Examples of such applications are dithering, chopping,and spread spectrum technologies.

Typically, the pseudorandom binary sequence is generated to approximatewhite noise characteristics. However, in some scenarios spectral shapingof the pseudorandom binary sequence may be desirable. While suchspectral shaping may be accomplished by filtering of a pseudorandombinary sequence having white noise characteristics, such digitalfiltering may result in an significant increase of circuit complexity.For example, the filtering may result in conversion of a single bitpseudorandom binary sequence to a multi-bit signal, and such multi-bitsignal may not be directly applicable for the intended purpose, e.g.,chopping by controlling a switch. Accordingly, a conversion of themulti-bit signal to a single bit signal may be necessary, which addscomplexity. In other cases, the multi-bit signal may be utilized assuch, but requires utilization of more complex components than in thecase of a single bit signal. For example, if the pseudorandom binarysequence is applied for dithering, utilization of a multi-bitdigital-to-analog converter (DAC) instead of a single bit DAC may berequired.

Accordingly there is a need for techniques which allow for efficientlygenerating a spectrally shaped pseudorandom binary sequence.

SUMMARY

According to an embodiment, a pseudorandom binary sequence having afirst sampling rate is generated. Based on the pseudorandom binarysequence, a digital filter generates a spectrally shaped pseudorandombinary sequence having a second sampling rate. The second sampling rateis equal to the first sampling rate multiplied by an integer upsamplingfactor of L>1. The digital filter comprises L filter branches consistingof a first subset of one or more filter branches and a second subset ofone or more filter branches. Each filter branch of the first subsetgenerates a binary output which is equal to an input of the filterbranch. Each filter branch of the second subset generates a binaryoutput which is inverted with respect to an input of the filter branch.

According to further embodiments of the invention, other devices ormethods may be provided. Such embodiments will be apparent from thefollowing detailed description in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a device according to an embodiment ofthe invention;

FIG. 2 schematically illustrates an implementation of device accordingto an embodiment of the invention, which is based on a polyphase digitalfilter;

FIG. 3 shows a device according to an embodiment of the invention, whichis based on a digital filter having a first order high pass filtercharacteristic;

FIG. 4 shows an FFT (Fast Fourier Transform) spectrum of a pseudorandombinary sequence without spectral shaping;

FIG. 5 shows an FFT spectrum of a pseudorandom binary sequence withspectral shaping using to a digital filter configuration as shown inFIG. 3;

FIG. 6 shows a device according to an embodiment of the invention, whichis based on a digital filter having a second order high pass filtercharacteristic;

FIG. 7 shows an FFT spectrum of a pseudorandom binary sequence withspectral shaping using to a digital filter configuration as shown inFIG. 6;

FIG. 8 shows a device according to an embodiment of the invention, whichis based on a digital filter having a band pass filter characteristic;

FIG. 9 shows an FFT spectrum of a pseudorandom binary sequence withspectral shaping using to a digital filter configuration as shown inFIG. 8; and

FIG. 10 shows a flowchart for schematically illustrating a method ofgenerating a spectrally shaped pseudorandom sequence according to anembodiment of the invention.

DETAILED DESCRIPTION

In the following, various embodiments will be described in detail withreference to the accompanying drawings. It should be noted that theseembodiments serve only as examples and are not to be construed aslimiting. For example, while embodiments with a plurality of features,other embodiments may comprise less features and/or alternativefeatures. Furthermore, features from different embodiments may becombined with each other unless specifically noted otherwise.

Embodiments as illustrated in the following relate to generation of aspectrally shaped pseudorandom binary sequence and correspondinglyconfigured devices, e.g., pseudorandom sequence generators or devicesequipped with a pseudorandom sequence generator. The spectrally shapedpseudorandom binary sequence may be based on an original pseudorandombinary sequence generated by a linear feedback shift register (LFSR).However, other kinds of pseudorandom sequence generators (PN-generators)could be utilized as well.

In the illustrated examples, a digital filter generates the spectrallyshaped pseudorandom binary sequence based on an original pseudorandombinary sequence. The original pseudorandom binary sequence has a firstsampling rate, and the spectrally shaped pseudorandom binary sequencehas a second sampling rate which is increased with respect to the firstsampling rate by an integer upsampling factor of L>1. The digital filterhas L filter branches which consist of a first subset of one or morefilter branches and a second subset of one or more filter branches. Eachfilter branch of the first subset is configured to generate a binaryoutput which is equal to an input of the filter branch. For example, ifthe input of the filter branch corresponds to either 1 or −1, the binaryoutput of the filter branch would correspond to 1 or −1, respectively.In another example, if the input of the filter branch corresponds toeither 1 or 0, the binary output of the filter branch would correspondto 1 or 0, respectively. Such behavior may be achieved by configuringthe filter branch to accomplish a multiplication by a filter coefficientof 1. Each filter branch of the second is configured to generate abinary output which is inverted with respect to an input of the filterbranch. For example, if the input of the filter branch corresponds toeither 1 or −1, the binary output of the filter branch would correspondto −1 or 1, respectively. Such behavior may be achieved by configuringthe filter branch to accomplish a multiplication by a filter coefficientof −1. In another example, if the input of the filter branch correspondsto either 1 or 0, the binary output of the filter branch wouldcorrespond to 0 or 1, respectively. Such behavior may be achieved byconfiguring the filter branch to accomplish a binary inversionoperation.

Due to the upsampling and the characteristics of the filter branches, itcan be achieved that the spectrally shaped pseudorandom binary sequenceis generated with the same bit length as the original pseudorandombinary sequence. For example, if the original pseudorandom binarysequence is a single bit signal, also the spectrally shaped pseudorandombinary sequence may be generated as a single bit signal. This allows forapplying the spectrally shaped pseudorandom binary sequence for directlycontrolling one or more switches, e.g., for the purpose of chopping asignal, or for utilizing the spectrally shaped pseudorandom binarysequence in connection with low complexity components, such as a singlebit DAC.

FIG. 1 schematically illustrates an example of a device operatingaccording to the above-mentioned principles. As illustrated, the deviceincludes a PN-generator 110, e.g., based on an LFSR. The PN-generator110 generates the original pseudorandom binary sequence x[k] at thefirst sampling rate, in FIG. 1 denoted by F_(SL). In the example of FIG.1, it is assumed that the original pseudorandom binary sequence x[k] isformed of samples having either a value of 1 or a value of −1.

Further, the device includes an upsampling stage 120. The upsamplingstage 120 receives the original pseudorandom binary sequence x[k] andperforms upsampling of the original pseudorandom binary sequence x[k] bythe upsampling factor L. This is accomplished by zero stuffing, i.e., bygenerating from the pseudorandom binary sequence x[k] an upsampledpseudorandom binary sequence x′[k] having the second sampling rate, inFIG. 1 denoted by F_(SH), which is increased by the upsampling factor L.The upsampled pseudorandom binary sequence x′[k] includes the samples ofthe original pseudorandom binary sequence x[k], separated by L−1 sampleshaving the value zero.

Further, the device includes a digital filter 150 operating at thesecond sampling rate F_(SH). As illustrated, the digital filter 150includes a series of delay registers 160 which receive the upsampledpseudorandom binary sequence x′[k]. In particular, a first delayregister 160 of the series receives the upsampled pseudorandom binarysequence x′ Pa delays it by one sample duration, and then passes it tothe next delay register 160 of the series, if present. The next delayregister 160 of the series receives the delayed upsampled pseudorandombinary sequence x′ Pa delays it by one sample duration, and then passesit to the next delay register 160 of the series, if present. Thissequential delaying of the upsampled pseudorandom binary sequence x′[k]continues until the last delay register 160 of the series is reached.The number of the delay registers 160 is L−1. Accordingly, at least onedelay register 160 is present. Depending on the filteringcharacteristics needed for obtaining a desired spectral shaping, thenumber of the delay registers 160 may be increased, and the upsamplingfactor L be adapted accordingly.

At the input of the first delay register 160 of the series, and at theoutput of each of the delay registers 160 of the series, a tap isprovided which feeds a corresponding filter branch of the digital filter150. In the illustrated example, the digital filter 150 provides Lfilter branches. Each filter branch includes a digital multiplier 170 toaccomplish a multiplication by a corresponding filter coefficient c_(i),with i=1, . . . , L. Outputs of the filter branches are fed to asummation stage 180, which sums the outputs of all filter branches togenerate the spectrally shaped pseudorandom binary sequence z[k]. Thespectrally shaped pseudorandom binary sequence z has the second samplingrate F_(SH).

As indicated in FIG. 1, the filter coefficients c_(i) are either 1 or−1. If the filter coefficient c_(i) of a certain filter branch is 1, theoutput of the filter branch is equal to the input of the filter branch,apart from any delay associated with the multiplication. If the filtercoefficient c_(i) of a certain filter branch is −1, the output of thefilter branch is inverted with respect to the input of the filterbranch, apart from any delay associated with the multiplication. Due tothe upsampling with zero stuffing, only one of the filter branchescontributes to each sample of the spectrally shaped pseudorandom binarysequence z[k]. Accordingly, the samples of the spectrally shapedpseudorandom binary sequence z[k] will have values of either 1 or −1,like in the original pseudorandom binary sequence z[k]. That is to say,both the original pseudorandom binary sequence x[k] and the spectrallyshaped pseudorandom binary sequence z[k] have the same bit length. Inparticular, both the original pseudorandom binary sequence x[k] and thespectrally shaped pseudorandom binary sequence z[k] are single bitsignals.

FIG. 2 schematically illustrates a further example of a device operatingaccording to the above-mentioned principles. Similar to the device ofFIG. 1, the device includes a PN-generator 110, e.g., based on an LFSR.The PN-generator 110 generates the original pseudorandom binary sequencex[k] at the first sampling rate, again denoted by F_(SL). Also in theexample of FIG. 2, it is assumed that the original pseudorandom binarysequence x[k] is formed of samples having either a value of 1 or a valueof −1.

Further, the device includes a digital filter 250. The digital filter250 is implemented as a polyphase filter. As illustrated, the digitalfilter 250 includes L filter branches which receive the originalpseudorandom binary sequence x[k] in parallel. That is to say, a sampleof the original pseudorandom binary sequence x[k] is simultaneously fedas input to each of the filter branches. Each filter branch includes adigital multiplier 270 to accomplish a multiplication by a correspondingfilter coefficient c_(i), with i=1, . . . , L. Outputs of the filterbranches are fed to a multiplexer 280, which sequentially selects theoutput of one of the filter branches. In the illustrated example, it isassumed that the multiplexer operates as a rotating switch. That is tosay, according to a periodic pattern the multiplexer 280 selects one ofthe filter branches after the other to generate the spectrally shapedpseudorandom binary sequence z[k]. The multiplexer 280 operates at thesecond sampling rate F_(SH), and the spectrally shaped pseudorandombinary sequence z[k] obtained at the output of the multiplexer 280 thushas the second sampling rate F_(SH). The multiplexer 280 thus alsoaccomplishes upsampling to the second sampling rate F_(SH), however inthis case without zero stuffing. As also indicated in FIG. 2, while themultiplexer 280 operates at the second sampling rate F_(SH), otherportions of the digital filter 250 only need to operated at the lowerfirst sampling rate F_(SL).

As indicated in FIG. 2, the filter coefficients c_(i) are either 1 or−1. If the filter coefficient c_(i) of a certain filter branch is 1, theoutput of the filter branch is equal to the input of the filter branch,apart from any delay associated with the multiplication. If the filtercoefficient c_(i) of a certain filter branch is −1, the output of thefilter branch is inverted with respect to the input of the filterbranch, apart from any delay associated with the multiplication. Due tothe operation of the multiplexer 280, only one of the filter branchescontributes to each sample of the spectrally shaped pseudorandom binarysequence z[k]. Accordingly, the samples of the spectrally shapedpseudorandom binary sequence z will have values of either 1 or −1, likein the original pseudorandom binary sequence x[k]. That is to say, boththe original pseudorandom binary sequence x[k] and the spectrally shapedpseudorandom binary sequence x[k] have the same bit length. Inparticular, both the original pseudorandom binary sequence x[k] and thespectrally shaped pseudorandom binary sequence x[k] are single bitsignals.

FIG. 3 shows an example of a device based on the architecture of FIG. 2,in which the digital filter 250 provides a first order high pass filtercharacteristic or first order differentiator characteristic. This isachieved by a configuration of the digital filter 250 with two filterbranches, i.e., L=2. The filter coefficient of the first filter branchis c₁=1, while the filter coefficient of the second filter branch isc2=−₁. This implements a transfer function of the digital filter 250which is given by

$\begin{matrix}{{{H(z)} = {\frac{Z\left\{ {y\lbrack k\rbrack} \right\}}{Z\left\{ {x\lbrack k\rbrack} \right\}} = \left( {1 - z^{- 1}} \right)}},} & (1)\end{matrix}$

where Z{x[k]} denotes the Z-transformation of x[k] and Z{y[k]} denotesthe Z-transformation of y[k].

FIG. 4 shows an example of an FFT spectrum of the original pseudorandombinary sequence x[k], with the frequency being denoted units of F_(SL).As can be seen, the FFT spectrum of the pseudorandom binary sequencex[k] substantially corresponds to a white noise characteristic, i.e.,has a substantially uniform distribution up to F_(SL).

FIG. 5 shows an example of an FFT spectrum of the spectrally shapedpseudorandom binary sequence y[k] as obtained when utilizing a digitalfilter configuration as illustrated in FIG. 3, i.e., a digital filterhaving a first order high pass characteristic. In FIG. 4, the frequencyis denoted units of F_(SH). As can be seen, the FFT spectrum showsdamping towards low frequencies.

FIG. 6 shows an example of a further device based on the architecture ofFIG. 2, in which the digital filter 250 provides a second order highpass filter characteristic. This is achieved by a configuration of thedigital filter 250 with four filter branches, i.e., L=4. The filtercoefficient of the first filter branch is c₁=1, the filter coefficientof the second filter branch is c₂=−1, the filter coefficient of thethird filter branch is c₃=−1, and the filter coefficient of the fourthfilter branch is c₄=1. This implements a transfer function of thedigital filter 250 which is given by

$\begin{matrix}{{H(z)} = {\frac{Z\left\{ {y\lbrack k\rbrack} \right\}}{Z\left\{ {x\lbrack k\rbrack} \right\}} = {\left( {1 - z^{- 1} - z^{- 2} + z^{- 3}} \right).}}} & (2)\end{matrix}$

FIG. 7 shows an example of an FFT spectrum of the spectrally shapedpseudorandom binary sequence y[k] as obtained when utilizing a digitalfilter configuration as illustrated in FIG. 6, i.e., a digital filterhaving a second order high pass characteristic. In FIG. 7, the frequencyis denoted units of F_(SH). As can be seen, the FFT spectrum showsdamping towards low frequencies. Further, from a zero point at F_(SH)/2,it can be seen that approximation of a second order differentiatorcharacteristic is not perfectly exact.

FIG. 8 shows an example of a further device based on the architecture ofFIG. 2, in which the digital filter 250 provides a second order bandpass filter characteristic. This is achieved by a configuration of thedigital filter 250 with four filter branches, i.e., L=4. The filtercoefficient of the first filter branch is c₁=1, the filter coefficientof the second filter branch is c₂=−1, the filter coefficient of thethird filter branch is c₃=1, and the filter coefficient of the fourthfilter branch is c₄=−1. This implements a transfer function of thedigital filter 250 which is given by

$\begin{matrix}{{H(z)} = {\frac{Z\left\{ {y\lbrack k\rbrack} \right\}}{Z\left\{ {x\lbrack k\rbrack} \right\}} = {\left( {1 - z^{- 1} - z^{- 2} - z^{- 3}} \right).}}} & (3)\end{matrix}$

FIG. 9 shows an example of an FFT spectrum of the spectrally shapedpseudorandom binary sequence y[k] as obtained when utilizing a digitalfilter configuration as illustrated in FIG. 8, i.e., a digital filterhaving a second order band pass characteristic. In FIG. 9, the frequencyis denoted units of F_(SH). As can be seen, the FFT spectrum shows apassband located around F_(SH)/8, with damping towards lower frequenciesand towards F_(SH)/4.

As can be seen from the examples of FIGS. 3 to 9, various kinds ofspectral shaping may be achieved by selecting different values of L andadapting the characteristics of the different branches. Here, it is tobe understood that similar results could also be obtained when utilizinga device based on the architecture of FIG. 1.

FIG. 10 shows a flowchart for illustrating a method which may beutilized to implement the above principles of generating a spectrallyshaped pseudorandom binary sequence. The method of FIG. 10 may forexample be applied for operating a device having an architecture asillustrated in FIG. 1 or 2.

At step 1010, a pseudorandom binary sequence is generated. Thepseudorandom binary sequence has a first sampling rate, e.g., theabove-mentioned sampling rate F_(SL). The pseudorandom binary sequencemay for example be generated by an LFSR.

At step 1020, upsampling is performed. This may be accomplished byperforming upsampling with zero stuffing on the pseudorandom binarysequence to generate an upsampled pseudorandom binary sequence havingthe second sampling rate, e.g., as described in connection with thearchitecture of FIG. 1. Further, this may be accomplished within apolyphase digital filter, by an multiplexer at the output of thepolyphase digital filter, e.g., as described in connection with thearchitecture of FIG. 2.

At step 1030, a spectrally shaped pseudorandom binary sequence isgenerated based on the pseudorandom binary sequence. The spectrallyshaped pseudorandom binary sequence has a second sampling rate equal tothe first sampling rate multiplied by an integer upsampling factor ofL>1. This is accomplished by a digital filter comprising L filterbranches consisting of a first subset of one or more filter branches anda second subset of one or more filter branches. Each filter branch ofthe first subset generates a binary output which is equal to an input ofthe filter branch. Each filter branch of the second subset generates abinary output which is inverted with respect to an input of the filterbranch. The digital filter may have a filter order of L−1.

The digital filter may be a polyphase filter and include a multiplexerwhich generates the spectrally shaped pseudorandom binary sequence bysequentially selecting one of the binary outputs of the filter branches.The multiplexer may also accomplish the upsampling of step 1020.

If at step 1020 an upsampled pseudorandom binary sequence having thesecond sampling rate is generated by upsampling with zero stuffing, theupsampled pseudorandom binary sequence may be received in a series ofL−1 delay registers of the digital filter, such as the delay registers160 of FIG. 1. Generating the spectrally shaped pseudorandom binarysequence may then also involve summing binary outputs of the filterbranches, such as by the summation stage 180 of FIG. 1. In such case,the input signals of the filter branches may be formed by the upsampledpseudorandom binary sequence and a respective output of each delayregister.

The spectrally shaped pseudorandom binary sequence may be a single bitsignal. In some scenarios, the pseudorandom binary sequence and thespectrally shaped pseudorandom binary sequence may be based on signalvalues selected from −1 and 1. In other scenarios, the pseudorandombinary sequence and the spectrally shaped pseudorandom binary sequencemay be based on signal values selected from 0 and 1.

If the spectrally shaped pseudorandom binary sequence is a single bitsignal, it may for example be applied in scenarios where at least oneswitch is controlled by the spectrally shaped pseudorandom binarysequence. Further, the spectrally shaped pseudorandom binary sequencemay allow for utilizing low complexity components when performing signalprocessing based on the spectrally shaped pseudorandom binary sequence.For example, for dithering purposes, the spectrally shaped pseudorandombinary sequence may be supplied to a single bit DAC.

Embodiments of the present invention, for example, may be applied tosystems in which dithering is used to reduce the amplitude of spuriousemissions caused by the harmonics of switching circuits. For example, inswitched mode power supplies and class-D amplifiers, embodiment PNgenerators may be used to dither switching signals. This ditheringspreads out the frequency content of high frequency harmonics andreduces RF interference. Such dithering may also allow switched-modepower supplies to operate at higher switching frequencies and still meetRF emission requirements compared to non-dithered systems. Embodiment PNsystems may also be applied to audio analog-to-digital converters anddigital-to-analog converters, such as sigma-delta converters to in orderto reduce limit cycle behavior. As mentioned above, by providing asingle-bit, spectrally shaped single-bit signal, dithering may beimplemented using circuitry that is lower in complexity, uses lesspower, and consumes less circuit board space and/or silicon area thandithering circuits that use multi-bit techniques.

It is to be understood that the above-described concepts and embodimentsare susceptible to various modifications. For example, the concepts maybe applied with various kinds of PN-generators and digital filterarchitectures. For example, in some embodiments, the various circuitcomponents, such as the disclosed PN generator 110, digital filter 250,delay registers 160, multiplier 270, summation stage 180, multiplexer280 and other components may be implemented using hardware-based digitallogic circuits known in the art. For example, components may beimplemented using standard cell or fully custom logic may be fabricatedon an integrated circuit. In some embodiments, the disclosed logicalfunctions may be implemented using hardware such as a digital signalprocessor and/or processor circuits such as a microprocessor,microcontroller or combinations thereof.

What is claimed is:
 1. A device, comprising: a pseudorandom sequencegenerator configured to generate a pseudorandom binary sequence having afirst sampling rate; and a digital filter configured to generate, basedon the pseudorandom binary sequence, a spectrally shaped pseudorandombinary sequence having a second sampling rate equal to the firstsampling rate multiplied by an integer upsampling factor of L>1, thedigital filter comprising L filter branches consisting of a first subsetof one or more filter branches and a second subset of one or more filterbranches, each filter branch of the first subset being configured togenerate a binary output which is equal to an input of the filterbranch, and each filter branch of the second subset being configured togenerate a binary output which is inverted with respect to an input ofthe filter branch.
 2. The device according to claim 1, wherein thedigital filter is a polyphase filter and comprises a multiplexerconfigured to generate the spectrally shaped pseudorandom binarysequence by sequentially selecting one of the binary outputs of thefilter branches.
 3. The device according to claim 1, comprising: anupsampling stage configured to perform upsampling with zero stuffing onthe pseudorandom sequence to generate an upsampled pseudorandom binarysequence having the second sampling rate.
 4. The device according toclaim 3, wherein the digital filter comprises a series of L−1 delayregisters configured to receive the upsampled pseudorandom binarysequence and a summation stage configured to sum the binary outputs ofthe filter branches, the input signals of the filter branches beingformed by the upsampled pseudorandom binary sequence and a respectiveoutput of each delay register.
 5. The device according to claim 1,wherein the spectrally shaped pseudorandom binary sequence is a singlebit signal.
 6. The device according to claim 5, wherein the pseudorandombinary sequence and the spectrally shaped pseudorandom binary sequencecomprise signal values selected from −1 and
 1. 7. The device accordingto claim 5, wherein the pseudorandom binary sequence and the spectrallyshaped pseudorandom binary sequence comprises signal values selectedfrom 0 and
 1. 8. The device according to claim 5, comprising: at leastone switch which is controlled by the spectrally shaped pseudorandombinary sequence.
 9. The device according to claim 1, wherein the digitalfilter has a filter order of L−1.
 10. The device according to claim 1,wherein the pseudorandom sequence generator comprises a linear feedbackshift register.
 11. A method, comprising: generating a pseudorandombinary sequence having a first sampling rate; and by a digital filtercomprising L filter branches consisting of a first subset of one or morefilter branches and a second subset of one or more filter branches,generating a spectrally shaped pseudorandom binary sequence based on thepseudorandom binary sequence, the spectrally shaped pseudorandom binarysequence having a second sampling rate equal to the first sampling ratemultiplied by an integer upsampling factor of L>1, each filter branch ofthe first subset generating a binary output which is equal to an inputof the filter branch, and each filter branch of the second subsetgenerating a binary output which is inverted with respect to an input ofthe filter branch.
 12. The method according to claim 11, wherein thedigital filter is a polyphase filter and comprises a multiplexer whichgenerates the spectrally shaped pseudorandom binary sequence bysequentially selecting one of the binary outputs of the filter branches.13. The method according to claim 11, comprising: performing upsamplingwith zero stuffing on the pseudorandom sequence to generate an upsampledpseudorandom binary sequence having the second sampling rate.
 14. Themethod according to claim 13, comprising: receiving the upsampledpseudorandom binary sequence in a series of L−1 delay registers of thedigital filter; and summing binary outputs of the filter branches, theinput signals of the filter branches being formed by the upsampledpseudorandom binary sequence and a respective output of each delayregister.
 15. The method according to claim 11, wherein the spectrallyshaped pseudorandom binary sequence is a single bit signal.
 16. Themethod according to claim 15, wherein the pseudorandom binary sequenceand the spectrally shaped pseudorandom binary sequence comprise signalvalues selected from −1 and
 1. 17. The method according to claim 15,wherein the pseudorandom binary sequence and the spectrally shapedpseudorandom binary sequence comprises signal values selected from 0and
 1. 18. The method according to claim 15, comprising: controlling atleast one switch by the spectrally shaped pseudorandom binary sequence.19. The method according to claim 11, wherein the digital filter has afilter order of L−1.
 20. The method according to claim 11, generatingthe pseudorandom binary sequence by a linear feedback shift register.